Warren said that researchers used to think that estrogen-like compounds from plants, called phytoestrogens, could possibly block the effect of estrogen in the body and perhaps reduce breast cancer risk. However, recent clinical studies have shown that women on diets high in soy phytoestrogens experience greater cell multiplication in the breast. There is concern over this effect since it could be a preliminary step in cancer formation by leading to the outgrowth of latent cancer cells.
In looking at studies on the use of supplements for menopause, Warren reports that the seven clinical trials using black cohosh were largely flawed but might suggest some effectiveness. However, little evidence exists for beneficial effects of red clover, and no benefit was reported for ginseng, evening primrose oil or dong quai. In the 16 studies that looked at soy supplements, twice as many studies reported no effect as those that reported a beneficial effect, and although kava kava has been reported to have some benefit, reports of serious side effects have led to it being banned for sale in the United States and elsewhere.
For premenstrual syndrome, several studies found some benefit in about half the women studied for evening primrose oil, but two of the best designed studies found no effect; ginkgo biloba extract was found to have no effect, while a beneficial response from chaste tree berry extracts was found in about half the women studied. For menstrual pain, two clinical trials have examined the Japanese herbal preparations toki-shakuyakusan and toki-shakuyakusan with shakuyaku-kanzoto; neither found a beneficial effect.
"Some herbal medicines show promise for the treatment of problems with menstruation and menopause and other conditions that